Maximum likelihood detection method using a sequence estimation receiver

ABSTRACT

The invention relates to a detection method using a receiver of a digital communication system for the detection of a symbol from a received signal, which signal is transmitted by a transmitter of the digital communication system, wherein the symbol is a selected symbol out of a predetermined set of symbols and wherein each symbol of the predetermined set comprises a sequence of chips wherein each of the chips is PSK-modulated according to a selected modulation code.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of European Patent Application No.01300298.5, which was filed on Jan. 15, 2001.

BACKGROUND OF THE INVENTION

The invention relates to a detection method using a receiver of adigital communication system for the detection of a symbol from areceived signal, which signal is transmitted by a transmitter of thedigital communication system, wherein the symbol is a selected symbolout of a predetermined set of symbols and wherein each symbol of thepredetermined set comprises a sequence of chips wherein each of thechips is PSK-modulated according to a selected modulation code.

Detection methods of this kind are known and find their applications ina variety of digital data communication systems. These datacommunication systems can for instance be wireless data communicationsystems. According to known methods, the receiver of the datacommunication system detects a symbol by detecting successively thechips of the symbol. This is referred to as chip-by-chip detection.After the detection of each chip, a correction signal for the Inter ChipInterference (ICI-effect) between neighbouring chips is generated andsubtracted from the received signal.

A first disadvantage of known methods is that detecting on achip-by-chip basis gives rise to some loss of coding gain with respectto detecting an entire symbol at one go (this is detecting onsymbol-by-symbol basis). A second disadvantage of known methods is thatno use is made of the a priori knowledge of the predetermined set.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a detection method whichyields an optimal coding gain by detecting an entire symbol at one go.It is also an object of the invention to provide a detection methodwhich uses a priory knowledge of the predetermined set. For this, thedetection method of the invention is characterised in that a set ofreference symbols is generated on the basis of the predetermined set ofsymbols and the channel impulse response between the transmitter and thereceiver, wherein each of the successive parts of the received signal,each part having the length of a symbol, is compared with each of thereference symbols, yielding a detected symbol for each part of thereceived signal. This detection method yields an optimal coding gainsince complete symbols are detected at one go. Furthermore, a prioryknowledge of the predetermined is used by only comparing the receivedsignal with reference symbols corresponding to symbols from thepredetermined set. The latter approach also saves processing power withrespect to a detection method wherein all possible combinations of chipsare compared with the received signal. A further advantage of the methodaccording to the invention is that the ICI-effect is maximallysuppressed. This is caused by the fact that the ICI-effect has beentaken into account implicitly in the generation of reference symbols.These reference symbols are created by convolving the symbols out of thepredetermined set with the channel impulse response between thetransmitter and the receiver.

A preferred embodiment of the method according to the invention ischaracterised in that a correction signal is generated on the basis ofthe detected symbol convolved with the channel impulse response, whereinthe correction signal is subtracted from the part of the received signalwhich succeeds the part of the received signal corresponding to thedetected symbol for suppressing the ISI-effect. The ISI-effect may occur(in an analogous way as the ICI-effect) as a consequence of TDS. In thedetection method of the invention, each time a part of the receivedsignal with the length of a symbol is corrected for the ISI-effect assoon as a symbol from the preceding part with the length of a symbol ofthe received signal is detected. The correction signal may be anestimated coefficient multiplied with the detected symbol convolved withthe channel impulse response.

A further embodiment of the method according to the invention ischaracterised in that the comparison between each of the parts of thereceived signal with each of the reference symbols is performed by acorrelator yielding a correlation value, wherein the correlation valueis corrected with half the energy of the reference symbol.

An alternative embodiment of the detection method of the invention ischaracterised in that the received signal is filtered by a filter whichyields a filter signal, wherein the filter is a matched filter to thechannel impulse response between the transmitter and the receiver,wherein each of the successive parts of the filter signal, each parthaving the length of a symbol, is compared with each of the symbols fromthe predetermined set of symbols yielding a detected symbol for eachpart of the filter signal. In this embodiment, the received signal is ina first step corrected for the channel impulse response betweentransmitter and receiver.

A further embodiment of the alternative detection method according tothe invention is characterised in that a correction signal is generatedon the basis of the detected symbol, wherein the correction signal issubtracted from the part of the filter signal which succeeds the part ofthe filter signal corresponding to the detected symbol for suppressingthe ISI-effect.

A still further embodiment of the alternative detection method accordingto the invention is characterised in that the comparison, between eachof the parts of the filter signal with each of the reference symbols, isperformed by a correlator yielding a correlation value, wherein thecorrelation value is corrected with half the energy of the referencesymbol.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, in which certain modes of carrying out thepresent invention are schematically shown for illustrative purposes:

FIG. 1 is a diagram schematically showing a receiver for performing adetection method according to the invention;

FIG. 2 presents a symbol comprising eight chips, wherein each chip isPhase Shift Keying Modulated;

FIG. 3 is a diagram schematically showing an alternative receiver forperforming a detection method according to the invention.

DETAILED DESCRIPTION

A receiver 2 of a digital communication system for receiving a signal 4and performing a detection, according to a detection method of theinvention, of a symbol from the received signal 4 is shown in FIG. 1.The signal 4 is transmitted by a transmitter of the digitalcommunication system to the receiver 2. The output 6 of the receiver 2is the detected symbol from the received signal 4. The detected symbolis a selected symbol out of a predetermined set of symbols. Each symbolof the predetermined set comprises a sequence of chips wherein each ofthe chips is PSK-modulated according to a selected modulation code. Anexample of a symbol 8 of the predetermined set comprising eight chips C₁up to and including C₈ is shown in FIG. 2.

The receiver 2 comprises a set of correlation blocks 10.1 up to andincluding 10.M for performing correlations between the received signal 4and reference symbols from a set of reference symbols, a decision block12 for selecting one of said reference symbols, a feedback block 14 forgenerating a correction signal 15 for suppressing the Inter SymbolInterference effect (ISI-effect) and an estimation block 16 forgenerating an estimation 18 of the channel impulse response between thetransmitter and the receiver on the basis of a reference signal 20.

In a first step of the detection method of the receiver 2, the receivedsignal 4 is correlated with symbols from a set of reference symbols.These correlations are performed by a set of M correlation blocks 10.m(m=1, . . . ,M). In this example, the m^(th) correlation blockcorrelates the received signal 4 with the M^(th) reference symbol S_(m)from the set of M reference symbols. The correlations are performedbetween successive parts of the received signal 4 wherein each part hasthe length of a symbol. This yields M correlation values 22.m (m=1, . .. ,M) per part of the received signal. Each of the correlation valuesmay be corrected with half the energy of the corresponding referencesymbol in order to make the correlation values mutually comparable.Next, these correlation values are the input to the decision block 12which selects the reference symbol corresponding to the largestcorrelation value. The detected symbol is the symbol from thepredetermined set of symbols which corresponds with the selectedreference symbol for the part of the received signal. The detectedsymbol is the output 6 of the receiver 2.

Time Delay Spread (TDS) is the effect wherein a received signal, whichhas been transmitted by a transmitter, has spread out in time withrespect to the signal before transmission. This results in that a partof the signal may end up in another part of the received signal. TheTDS-effect may occur as a result of multi-path in the channel betweentransmitter and receiver. The Inter Chip Interference (ICI-effect) isthe result of the TDS-effect wherein a chip smears out over aneighbouring chip. The Inter Symbol Interference (ISI-effect) is theresult of the TDS-effect wherein a symbol smears out over a neighbouringsymbol. The receiver 2, performing a detection method according to theinvention, effectively suppresses the ICI-effect since entire symbolsare detected at one go by comparing entire symbols with referencesymbols (this is called detection on a symbol-by-symbol basis). Sincethe reference symbols are convoluted with the channel impulse response,the ICI-effect is automatically accounted for.

For the suppression of the ISI-effect in the detection method performedby the receiver 2, a feedback loop is used comprising the feedback block14. The feedback block 14 calculates a correction signal 15 for thereceived signal 4. This correction signal 15 is a function of thedetected symbol. This function may be an estimated coefficientmultiplied with the detected symbol convolved with the channel impulseresponse between the transmitter and the receiver. The correction signal15 is subtracted from the part of the received signal which succeeds thepart of the received signal corresponding to the detected symbol.

The M reference symbols S_(m) (m=1, . . . ,M) are obtained byconvoluting the symbols from the predetermined set with the channelimpulse response between the transmitter and the receiver. Thisimplicates that the symbols from the set of reference symbols have to bere-calculated regularly and at least every time the channel impulseresponse has changed. The estimation 18 of the channel impulse responseis performed by the estimation block 16 on the basis of the referencesignal 20. The estimation 18 is fed to the correlation blocks 10.m (m=1,. . . ,M). Then, the m^(th) correlation block calculates the referencesymbol S_(m) (m=1, . . . ,M) which is used for correlations to beperformed by the m^(th) correlator.

An alternative receiver 24 of a digital communication system forreceiving a signal 4 and performing a detection method according to theinvention of a symbol from the received signal 4 is shown in FIG. 3. Thesignal 4 is transmitted by a transmitter of the digital communicationsystem to the receiver 24. The output 6 of the receiver 2 is thedetected symbol from the received signal 4. The receiver 24 comprises amatched filter 26 on the channel between the transmitter and thereceiver, a correlator-bank 28 comprising a set of correlators, acorrection block 30 for generating correction terms 31 for the output ofthe correlator-bank 28, a decision block 32 for selecting a detectedsymbol from the predetermined set of symbols and a feedback-block 34 forgenerating a correction signal 36 for suppressing the ISI-effect.

The first step in the receiver 24 is the filtering of the receivedsignal 4 by the channel matched filter block 26, resulting in the filtersignal 27. The filter signal 27 is fed to a correlator-bank 28comprising M correlators, wherein successive parts of the filter signal27 are correlated with each of the M symbols from the predetermined setof symbols. This yields an output 29 of the correlator-bank 28 of Mcorrelation values per part of the filter signal 27. The m^(th)correlation value results from the correlation between the part of thefilter signal 27 and the m^(th) symbol from the predetermined set ofsymbols. Each of the correlation values is corrected by the correctionblock 30 with a correction value 31 in order to generate correlationvalues which are mutually comparable. In this example, the correctionvalue is half the energy of the corresponding symbol. The decision block32 selects the symbol from the predetermined set of symbols whichcorresponds to the largest correlation value; this selected symbol isthe detected symbol 6. Since the detection of the symbol is performed ona symbol-by-symbol basis, the ICI-effect is effectively suppressed (inan analogous way as described hereinbefore in relation to receiver 2).The suppression of the ISI-effect is suppressed with a feedback loopwherein the feedback block 34 generates a correction signal 36 on thebasis of the detected symbol 6 (in an analogous way as describedhereinbefore in relation to receiver 2).

It will be clear to those skilled in the art that the invention can bepracticed otherwise than as specifically illustrated and describedwithout departing from its spirit or scope. For example, amplitudemodulation can be used in combination with PSK-modulation.

1. A detection method using a receiver of a digital communication systemfor the detection of a symbol from a received signal, which signal istransmitted by a transmitter of the digital communication system,wherein the symbol is a selected symbol out of a predetermined set ofsymbols and wherein each symbol of the predetermined set comprises asequence of chips wherein each of the chips is PSK-modulated accordingto a selected modulation code, wherein the method comprises: generatinga set of reference symbols on the basis of the predetermined set ofsymbols and a channel impulse response between the transmitter and thereceiver; comparing each of the successive parts of the received signal,each part having the length of a symbol, with each of the referencesymbols, yielding a detected symbol for each part of the receivedsignal; and generating a correction signal on the basis of the detectedsymbol convolved with the channel impulse response, wherein thecorrection signal is subtracted from the part of the received signalwhich succeeds the part of the received signal corresponding to thedetected symbol for suppressing the ISI-effect.
 2. The method accordingto claim 1, wherein the comparison between each of the parts of thereceived signal with each of the reference symbols is performed by acorrelator yielding a correlation value, wherein the correlation valueis corrected with half the energy of the reference symbol.
 3. Adetection method using a receiver of a digital communication system forthe detection of a symbol from a received signal, which signal istransmitted by a transmitter of the digital communication system,wherein the symbol is a selected symbol out of a predetermined set ofsymbols and wherein each symbol of the predetermined set comprises asequence of chips wherein each of the chips is PSK-modulated accordingto a selected modulation code, wherein the method comprises: filteringthe received signal with a filter which yields a filter signal, whereinthe filter is a matched filter to the channel impulse response betweenthe transmitter and the receiver; comparing each of the successive partsof the filter signal, each part having the length of a symbol, with eachof the symbols from the predetermined set of symbols yielding a detectedsymbol for each part of the filter signal; and generating a correctionsignal on the basis of the detected symbol, wherein the correctionsignal is subtracted from the part of filter signal which succeeds thepart of the filter signal corresponding to the detected symbol forsuppressing the ISI-effect.
 4. The method according to claim 3, whereinthe comparison, between each of the parts of the filter signal with eachof the reference symbols, is performed by a correlator yielding acorrelation value, wherein the correlation value is corrected with halfthe energy of the reference symbol.